French Patent Application No. 9408420, filed Jul. 7, 1994, which corresponds to U.S. patent application Ser. No. 08/497,380, filed Jun. 29, 1995, now U.S. Pat. No. 5,623,368 describes a process for manufacturing a network of microlenses using a mold. In this process the cells of the mold have a depth greater than the depth of the convex part of the microlenses which are formed. Under the effect of the pressure exerted on the plate, the deformable optical material projects into the cells, taking on the convex shape of convex microlenses, without the convex surfaces of these microlenses coming into contact with the bottom of the cells, thereby preserving the optical finish of these convex surfaces.
The networks of microlenses thus obtained have various applications, particularly in image sensors or reproducers and in particular in video image projectors operating by projecting light through a two-dimensional matrix of cells with liquid crystals displaying the image to be projected. The luminous efficiency of such projectors is improved by focusing the light from a source on the useful areas of the cells, which is obtained by a network of microlenses. It is therefore appropriate for the opening of these lenses to be as large as possible, in order to allow the maximum amount of light to pass through. For this purpose, the cells hollowed in the mold, on the surface of the mold, have polygonal forms which are closely imbricated with respect to one another and not less compact circular forms of imbrication.
Mechanical machining and reactive ion etching methods have been used heretofore to form molds having circular mold cavities for making microlens arrays.
The present invention, on the other hand, deals with the forming of molds having generally polygonal mold cell shapes, preferably using chemical etching techniques. In the manufacture of such a mold by chemical etching through a mask which conforms to the network of cells to be formed, one observes that the smaller the dimensions of the cells, the more difficult it is to obtain cells which open on the surface of the mold with the anticipated polygonal contour: square, rectangular, pentagonal, hexagonal, etc. In particular, one typically observes a rounding of what would otherwise be rectilinear sides of the polygonal cell, resulting in a more circular, rather than polygonal, shape of the opening of the cell on the plate, thereby reducing the opening of the microlenses which are formed. This phenomenon is particularly perceptible in the case of microlenses which are less than 100 .mu.m in diameter.
The present invention therefore aims to provide a process for manufacturing a mold consisting of a periodic network of mold cells on the surface of a plate, the cells having generally polygonal shapes defined by generally rectilinear cell side boundary contours.
Another problem which occurs in pressing lenses from polygonal mold cells is that the convex surfaces of the microlenses formed can deviate from the desired sphericity because the optical material is being pressed into a mold having other than a circular shape. The present invention therefore also aims to improve the sphericity of the optical surfaces of microlenses obtained by pressing a deformable optical material against a network of cells having generally polygonal edges.